1. Field of the Invention
The present invention relates to a structure for examining and confirming whether a circuit protective element arranged on a circuit board normally operates or not.
2. Description of the Related Art
In a conductive circuit formed in a pattern on an electronic circuit board, a thermistor having a positive resistance temperature characteristic (hereinafter referred to as a PTC thermistor) is used as a circuit protective element for shutting off an overcurrent. The PTC thermistor is such a resistor that a no-load resistance value sharply increases with a rise of temperature. In other words, when an overcurrent flows in the conductive circuit connected to the PTC thermistor, the temperature of the PTC thermistor itself is raised by heat generated at the conductive circuit. The rise of the heat will sharply increase the resistance value of the PTC thermistor, and the PTC thermistor controls a current value of the current flowing through the conductive circuit, which is hereinafter called as xe2x80x9cnormal operationxe2x80x9d. The circuit board will be thus protected.
However, if a product installed with the PTC thermistor is defective, that is, the PTC thermistor does not operate normally, it will not be active when the overcurrent flows into the conductive circuit. In the worst case, there is an anxiety that the circuit board may fire. Therefore, an examination must be conducted prior to a shipment of the product, as to whether the PTC thermistor on the circuit board normally operates or not.
Conventionally, the following methods have been proposed as methods for supplying a large current for examination to the PTC thermistor in the product: namely, as shown in FIG. 12, a first method of supplying the examining large current from terminals 82 which are soldered to a conductive circuit 81 on a circuit board 80, or a second method of supplying the examining large current directly to lead terminals 84 of a PTC thermistor 83, or a third method of supplying the examining large current by directly bringing a test probe 86 into contact with conductive plates 85. The large current for examining purpose (for example 100 ampere) is a current larger (more intense) than the overcurrent.
However, in the first method, current value of the large current cannot substantially exceed an admitted current value of the terminals 82. Therefore, there has been a drawback that it takes a long time for the PTC thermistor 83 to start the normal operation, and the examining period will be longer. In the second method, it is sometimes impossible to directly bring the probe 86 in contact with the lead terminal 84 because of a position where the PTC thermistor 83 is arranged. In the third method, the conductive plate 85 is too thin to bear the large current.
In view of the foregoing, it is an object of the present invention to provide a structure for examining a circuit protective element which can reliably examine and confirm a normal operation of the circuit protective element (PTC thermistor), and enables high grade products to be manufactured at a low cost.
In order to attain the above described object, there is provided according to the present invention, an examining structure for a circuit protective element comprising a plurality of conductive plates of a conductive circuit which is formed in a pattern on a circuit board, a pair of lead terminals provided in the circuit protective element and respectively connected to the conductive plates, a test probe for supplying a large current to the lead terminals in order to examine a normal operation of the circuit protective element, and a test pad soldered to the conductive plate which is connected to the lead terminal, the test pad being adapted to be in contact with the test probe, thereby constituting a probe contact part having a thickness larger than the conductive plate.
According to another feature of the present invention, the test pad is smaller than a surface of the conductive plate and is positioned near said lead terminal.
According to a further feature of the invention, the test pad includes a plurality of contact members having the same shape thereby to keep a contact area with the test probe uniformly.
According to a still further feature of the invention, the contact members are uniformly arranged.
According to a still further feature of the invention, the contact members are uniformly arranged on one face of the conductive plate, and include solders attached directly to and projecting from the one face.
According to another feature of the invention, the solders includes a solder filled in a through hole formed in the conductive plate thereby to form the solder at an opening of the through hole.